It will be recalled with relation to FIG. 1 that the geolocalization of a stationary target consists in determining the absolute position of the target on the ground, represented by a point P, from the differences in x-coordinate X and y-coordinate Y between the target and the projection C0 on the ground of the aircraft, called the carrier. The position C0 is provided by the navigation system of the aircraft, in general with an error. The angles R and G are respectively the roll angle and the azimuth angle of the line of sight of the target in a terrestrial reference frame. The angles G and R notably depend on the attitude of the aircraft and on the orientation of the line of sight relative to that of the aircraft. They are provided by the navigation system of the aircraft.
FIG. 2 provides a vertical section of FIG. 1 when Y is zero. Except where explicitly mentioned, the remainder of the text is based on this case, without impinging on the generality. This is because the angle G is generally small under the usual operating conditions.Hence: X=h·tan R  (1)in which h is the height of the aircraft vertically above C0, and is provided by the navigation system.
The result obtained for X is sensitive to errors in h and R, above all for grazing angles of incidence, i.e. when R is in the vicinity of 90°. Hence, to a first order:εX=(∂X/∂h)εh+(∂X/∂R)εR εX/X=εh/h+εR(1+tan2 R)/tan R 
For grazing angles of incidence, 1/tan R <<tan R, finally yielding:εX/X=εh/h+εR·tan R εX, εh, εR being respectively the errors in X, h and R.
Hence, for example, for h=1000 m, εh=0 and X=30 km, an error of 1 mrad in R leads to a geolocalization error of 900 m, i.e. of 3%, which is unacceptable from an operational point of view.
This is because in the case of an application to a reconnaissance system the precision of the geolocalization is a fundamental specification of this system. In the case of a prepared mission, the system must acquire data in an area of the operational theater, defined by its absolute coordinates and designated the target area. An error in geolocalization leads to an increase in the size of the area to be covered in order to be sure that it contains the target area. In addition, it is necessary to know the distance between the sensor and the points to be imaged in order to guarantee the quality of the image (blurring by motion).
Furthermore, a precision in R to within 1 mrad is very difficult to achieve, in particular if the sensor is mounted on a pod which itself moves relative to the aircraft: in this case, an error of several mrad is usual.
In addition, when the ground is uneven and the difference in height between the target and C0 is not known, the formula (1) leads to an even larger error in the measurement of X, as illustrated in FIG. 3. Measured X is obtained in place of X.
Furthermore, atmospheric refraction introduces an additional error in the measurement of X, as illustrated in FIG. 4, yielding measured X in place of X. This error may be around 900 meters.
The aim of the invention is to determine by means of a passive optronic sensor the x-coordinate X with better precision, around 1% for example, notably when the difference in height between the target and C0 is not known and/or in the presence of possible atmospheric refraction.